1. Field of the Invention
The invention relates to a CT X-ray apparatus, and more particularly to a scan and reconstruction method, device, and computer-readable medium when a dynamic scan is conducted in a CT X-ray apparatus radiating a cone beam.
The present invention includes use of various technologies referenced and described in the references identified in the appended LIST OF REFERENCES and cross-referenced throughout the specification by boldface numerals in brackets corresponding to the respective references, the entire contents of all of which are incorporated herein by reference.
2. Discussion of the Background
Cone-beam computed tomography (CT) reconstructs the interior of an object of interest or patient O from two-dimensional projections PD of X-rays transmitted through the object of interest or patient, as illustrated in FIG. 1c. An X-ray source FP and an X-ray detector D are arranged in a number of different positions so that X-rays transmitted through the object of interest O are received at the detector D. The detector D, either alone or in conjunction with other devices, generates image data for each position of the source and/or detector. The image data is then stored, manipulated, and/or analyzed to reconstruct the interior of the object. In a cone-beam CT system, the detector D is in the form of an array of X-ray sensing elements.
An approximate reconstruction method, the so-called Feldkamp reconstruction method, has been described [1, 2]. In Feldkamp reconstruction, the focal point of an X-ray is moved along an ideally circular orbit P around a completely immobile object or patient, and a volume V is reconstructed by using the collected cone beam projected data, as illustrated in FIGS. 1a and 1b. The Feldkamp reconstruction can be generally expressed as shown in Eq. 1, where the function FC (xc2x7) indicates that projection data pC obtained along a circular orbit is processed to yield the Feldkamp reconstruction of the interior volume of the patient or object of interest V, where:
V|t=t1=FC(pC|t=t1)xe2x80x83xe2x80x83Eq.(1)
FC(xc2x7): method of processing projection data obtained along a circular orbit
pC|t: projection data at time period t obtained along a circular orbit
V|t: volume to be reconstructed as it existed at time period t
t1: data collection time period, i.e., an imaging time period
As seen above, projection data along a circular orbit P is collected over a finite period of time t1 that is required for translating the X-ray source and detector, as well as integrating the received X-ray intensity. Since a single reconstruction of the volume V requires the use of data collected at different times within the period t1, any shifting of the patient or object of interest during imaging quickly degrades image quality.
Even when the patient or object is completely immobile, when the cone angle becomes large, image artifacts in Feldkamp reconstruction are increased and the image quality deteriorates. In order to avoid this deterioration in image quality, other scan and reconstruction methods have been proposed [3a, 3b, 4]. A strict reconstruction method has been described in which the focal point of the X-rays is moved along linear and circular orbits P around a completely immobile patient or object of interest, and reconstruction is conducted by using the collected cone beam projected data [3a, 3b, 4], as illustrated in FIGS. 2a, 2b, 3a and 3b. As illustrated in FIG. 2b and hereinafter, orbits P where data is collected are denoted by arrows in bold type. These types of image reconstructions can be generally expressed as shown in Eq. 2, where the function FC(xc2x7) indicates that projection data pC collected along the circular orbit is processed in a certain manner, and FL(xc2x7) indicates that projection data PL collected along a linear orbit is processed in a certain manner. Although the function designation FC(xc2x7) is the same as the function designation used as in Eq. 1, the two functions are not necessarily the same. Thus, FC(xc2x7) in Eq. 2 is not necessarily the Feldkamp reconstruction, but rather only denotes the processing of data obtained along a xe2x80x9ccircular orbit.xe2x80x9d
V|t=t1=FC(pC|t=t1)+FL(pL|t=t0)xe2x80x83xe2x80x83Eq. (2)
FC(xc2x7): method of processing projection data obtained along a circular orbit
FL(xc2x7): method of processing projection data obtained along a linear orbit
pC|t: projection data at time period t obtained along a circular orbit
pL|t: projection data at time period t obtained along a linear orbit
V|t: volume to be reconstructed as it existed at time period t
Although the volume reconstructed by this method displays reduced deterioration in image quality, a problem still arises due to the finite times required for data collection. Typically, the linear orbit is scanned before (FIGS. 2a and 2b) or after (FIGS. 3a and 3b) the circular orbit. Moreover, since the berth that supports the patient or object is translated between the scan plane of the circular orbit and the scan starting position of the linear orbit (in the appropriate direction), an additional delay is required. This is indicated in Eq. 2 by the fact that the projection data obtained along a circular orbit pC is obtained over a time period t1, whereas the projection data obtained along a linear orbit pL is obtained over a time period t0. As a result, relatively rapid movements that occur within time periods shorter than the sum of t1 and t0 degrade image quality, and only very slow, intermittent movements can be resolved with this scan method, as illustrated in FIG. 5.
For the sake of convenience, the time period for collecting data along a (full or partial) circular orbit will hereinafter be referred to as t1, t2, . . . tn. Likewise, the time period for collecting data obtained along another (e.g., linear and/or helical orbit) will be referred to as t0, regardless of which time period actually occurred first.
Other researcher have attempted to address the problem of relatively rapid (or continuous) movement during imaging by implementing cone-beam CT using projection data obtained from along a partial orbit of the object or patient. Such partial orbits are capable of providing complete image data for reconstruction of the interior of an object since many views in a complete circular orbit are redundant, i.e., the image data provide little or no new information. For example, if the object of interest is immobile and the system is ideal (i.e., no noise), switching the location of the source and detector will provide no new information along the ray through the axis even though image data from a second view has been collected.
A method for reconstruction of one particular partial orbit, namely an orbit that covers the xe2x80x9cminimal complete data setxe2x80x9d has been described in [6]. The xe2x80x9cminimal complete data setxe2x80x9d spans more than one half of a complete orbit. Namely, it spans 180xc2x0 plus the maximum fan angle 2xcex3m, where the maximum channel angle xcex3m is the largest angle of a ray emitted by the X-ray source that is received at the X-ray detector relative to the ray emitted from the source that passes through the axis of rotation of the X-ray source and detector.
Another method for the reconstruction of a partial orbit is described [8].
As illustrated in FIGS. 4a and 4b, plural partial and/or complete circular orbits can be excised from a continuous circular orbit P. As used herein, a xe2x80x9ccontinuous circular orbitxe2x80x9d need not extend into perpetuity, but rather indicates that several staggered partial and/or complete circular orbits can be excised from the scan. The xe2x80x9ccontinuous circular orbitxe2x80x9d illustrated in FIG. 4a and hereinafter is denoted by the undashed potion extending the circular orbit P beyond a single revolution. Once again, this is for illustrative purposes only, since a xe2x80x9ccontinuous circular orbitxe2x80x9d as used herein can actually span an angle smaller than one complete revolution if partial circular orbits are used for reconstruction. The collection times t1, t2, . . . tn of the excised portions are staggered in time at relatively short increments so that relatively quicker and continuous movements can be imaged. However, reconstruction when the cone angle is large still suffers from the image degradation described above. When linear scans are inserted between even partial orbits as illustrated in FIGS. 5a and 5b, the resultant images still suffer under the same delays described in Eq. 2 and are only able to resolve slow, intermittent movements.
A need to resolve continuous and/or relatively rapid changes with time of the volume V without deterioration in image quality thus exists.
Accordingly, one object of this invention is to develop a method, system, and computer-readable medium that resolve continuous and/or relatively rapid changes with time of the volume V without deterioration in image quality.
This and other objects of the invention are realized through a method, system, and computer-readable medium that, in one embodiment of this invention, combine a subset of the projection data collected along a continuous circular orbit with projection data collected along a different orbit to reconstruct the volume V substantially as it was when the subset of the projection data was collected. In one embodiment, the different orbit is a linear or a helical orbit. Further, staggered subsets of the projection data collected along a continuous circular orbit can also be used in some embodiments, as can further projection data collected along a linear orbit to resolve continuous and/or relatively rapid changes of a volume V in some embodiments.